Multilayer heterostructures for application in oleds and photovoltaic devices

ABSTRACT

This invention relates to a supported polymer heterostructure and methods of manufacture. The heterostructure is suitable for use in a range of applications which require semiconductor devices, including photovoltaic devices and light-emitting diodes.

This application is a divisional of application Ser. No. 14/557,493,filed Dec. 2, 2014, which is a divisional of application Ser. No.12/744,800 filed Jan. 11, 2011, now U.S. Pat. No. 8,927,383, which inturn is the U.S. national phase of International Application No.PCT/GB2008/003965 filed Nov. 28, 2008, which claims the benefit of U.S.Provisional No. 60/990,636 filed Nov. 28, 2007, the entire contents ofeach of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the field of polymer thin filmtechnology and devices such as organic electronic devices, plasticelectronic devices and organic semiconductor devices. In particular, itconcerns organic polymer heterostructures, e.g. organic polymersolvent-processable heterostructures which may contain one or morediffuse or abrupt interfaces. The heterostructures may benanostructured. The heterostructures may comprise a crosslinkablematerial. The present invention also relates to methods of making saidheterostructures.

BACKGROUND OF THE INVENTION

High-quality semiconductor heterostructures, which are junctions of twodissimilar materials in contact, are crucial to the proper operation ofmany organic semiconductor devices, including light-emitting diodes andphotovoltaics. The materials in contact may differ in one or more of thefollowing opto-electronic properties: the hole and electron transportlevels, refractive indices, excited state energy levels.

By selecting the appropriate materials which are in contact, one canform e.g., a charge-confinement interface which transmits carriers ofone sign but blocks carriers of the opposite sign. This may be achievedby adopting at least one of the following: the appropriate energyoffsets in the hole and electron transport levels; a charge-injectinginterface for efficient injection of charge of one sign by having agraded energy level; an exciton-confinement interface to prevent theexciton from wandering into the neighbouring layer by imposing a higherenergy there; a photon-confinement interface that prevents photons fromtravelling away into the neighbouring layer by imposing the conditionfor total internal reflection at the interface; acharge-carrier-generation interface to create electrons and holes uponabsorption of a photon by having the suitable offsets in both theelectron and hole energy levels for exciton dissociation; a cascaded setof energy levels for these electrons and holes for even more efficientcharge-carrier generation; or a charge-carrier-recombination interfaceto create excitons by the capture of electrons and holes electricallyinjected into the device by having the suitable offsets in both theelectron and hole energy levels for exciton generation.

An organic light emitting diode (OLED) consists of a cathode, anemissive layer, and an anode in a sandwich structure. The anode usuallyconsists of a transparent indium tin oxide (ITO) substrate coated with alayer of conducting polymer. The emissive layer consists of electrontransporting, hole transporting and emissive materials. These materialsmay be molecules, oligomers or polymers or segments of polymers, ornanocrystals, or nanowires and nanosheets, with a □-□* gap of 1-4electronvolts (eV). The cathode usually consists of a low work functionmaterial such as calcium, or a combination of an insulator such aslithium fluoride and a metal such as aluminium.

When a negative bias is applied to the cathode and is larger than thebuilt-in potential, electrons are injected into the electrontransporting material, while holes are injected into the holetransporting material. The electron transporting and hole transportingmaterials may be the same or different. The electrons are transportedinto the lowest unoccupied molecular orbital (LUMO) of the emissivematerial, and holes are transported into the highest occupied molecularorbital (HOMO) of the emissive material. The recombination of theseelectrons and holes gives a photon with energy corresponding to theLUMO-HOMO gap.

An organic photovoltaic (OPV) device consists of an electron collector(cathode), an absorption and charge-generating layer and a holecollector (anode). The electron collector usually consists of a low workfunction material such as calcium or aluminium. The absorption andcharge-generation layer consists of absorbing materials that absorb thelight to give an exciton state, charge dissociation materials thatdissociate this exciton state to give the electron and hole, and chargetransporting materials that transport the electrons and holes away fromthe dissociation sites. These materials may be the same or different.They may be molecules, oligomers or polymers or segments of polymers, ornanocrystals or nanowires or nanosheets, with a □-□* gap of 1-4electronvolts (eV).

Upon absorption of a photon with energy similar to the absorption gap ofthe organic material, an exciton is formed. Excitons arecoulombically-bound electron-hole pairs. Excitons need to be separatedinto electrons and holes, and collected at the cathode and anoderespectively. The efficiency of separation of these photo-excitedexcitons into free electrons and holes is therefore critical to theefficiency of OPVs. This separation is achieved at the interface betweentwo materials with appropriate energy offsets in both the LUMO and HOMOlevels and is crucial for the efficient dissociation of the excitons toprovide efficient OPVs. One of these materials is a hole transportingmaterial, and the other is an electron transporting material. Holes andelectrons are therefore respectively transported through these materialsto the respective electrodes to be collected.

For a number of applications, particularly forcharge-carrier-recombination in light-emitting devices, and forcharge-carrier-generation in photovoltaics, it is desirable to have adistributed heterostructure, as opposed to a planar heterostructure. Aplanar heterostructure is flat. A distributed heterostructure creates alarge interfacial area between the two materials in contact, by having“fingers” of one material in contact with the other material or onematerial embedded in the other, so that the two or more materials are inintimate contact. Having a large heterostructure interfacial area canimprove e.g., charge-carrier generation efficiency in photovoltaicdevices.

It is further desirable for the two charge-conducting materials thatcontinuous paths exist between all locations in the hole transportingmaterial to its proper contact (positively-biased contact for injectionof holes in light-emitting diodes and negatively-biased contact forcollection of holes in photovoltaics), and likewise for the electrontransporting material. In addition, such paths should preferentially liealong the most direct route; otherwise the transport of these carrierswould be obstructed, and the resistance of the device increasesundesirably.

Therefore, for these applications, the heterostructure not only has tohave a large surface area but its morphology should ideally be columnar(i.e., the shapes, such as either voids or columns, pass through thethickness of the film), which may be referred to herein as the “columnardistributed heterostructure”. Columnar distributed heterostructures arewidely expected to be beneficial, but so far their fabrication hasproved problematic. Controlling the morphology of such structures duringformation is not straightforward.

In particular, for organic photovoltaic devices, the required lateraldimensions of the heterostructure in the directions along the plane ofthe film (i.e., the lateral length scale of the distributedheterostructure) is related to the exciton diffusion length scale.Absorption of light creates an exciton that diffuses about in thematerial. It is essential that this exciton can reach thecharge-carrier-generating interface within its lifetime, in order toproduce electrons and holes that can be separated. For typical excitondiffusion length scales of a few to tens of nanometers, it is useful tohave the lateral length scale of similar dimensions, so that themajority of the excitons generated can reach the dissociation interface.With the charge-carriers so created, it is then essential for them to betransported to the respective collection electrodes. Up till now, it hasproven difficult to fabricate such heterostructures of such fineness ina suitable morphology. It is one of the objectives of this invention toprovide ways to achieve this without resorting to use of an electronbeam or other lithographic methods.

It has been shown by Friend and co-workers (Nano. Lett. 2002, 2,1353-1357), and others, that when two semiconducting polymers areco-dissolved in a common solvent, and the mixture deposited to give afilm by spin-coating or ink jet printing, the film formed will benaturally phase-separated. The character and length scale of the phaseseparation depends amongst other factors on the solvent and itsevaporation rate (influenced e.g., by heating or the presence of a highvapor pressure of the solvent). These are useful parameters in providinga degree of control over the phase separation length scale andmorphology, and hence of the distributed heterostructure, which has beenshown to be useful for photovoltaic applications.

The typical phase separation length scale is of the order of a fewmicrometers to tens of micrometers. Although the solvent and itsevaporation rate can influence this somewhat, it is ultimately stronglyrelated to the character of the polymers. Very fine heterostructuresbelow 1 micron in lateral length scale cannot readily be expected,particularly if the polymers are sufficiently incompatible that theyphase separate out early in the solution drying process. In this manner,a hierarchy of phase separation occurs in both the lateral and verticaldimensions, possibly forming isolated phases within the polymer thinfilm, which often leads to a so-called “Russian doll” morphology inwhich material A for example is completely occluded in material B.

As a result, the connectivity is broken; holes, for example, cannot flowout of the occluded phases of the hole-conducting material. Furthermore,it is possible for certain polymers to develop “wetting” layers so thatthey may form a partial or complete overlayer or underlayer at theinterfaces of the polymer thin film but not necessarily in the mostdesirable way. In the context of photovoltaic devices, this will meanthat a significant fraction of the charges generated within these phasescannot successfully reach the electrodes, thereby limiting theefficiency of the photovoltaic devices.

It has also been shown by Steiner and co-workers (Nature, 1998, 391,877-879) that the phase separation of polymer blends can be spatiallyinfluenced and directed by patterns pre-formed on the substrate. Suchpatterns include patterns in the surface energy, created e.g. bychemical reaction through a pattern-generating method such as contactprinting, or by photolithography. It has further been shown that polymerblends have preferred phase-separation length scales of the order of afew to tens of microns, and application of the phase-separation patternto the underlying chemical pattern is not possible if the length scalesare very different. These methods require the formation of a chemicalpre-pattern on the substrate, and then phase separation of the desiredpolymer combination over the pattern.

Bulk distributed heterostructures are known in the art and are used inpolymer-based photovoltaic devices. Bulk distributed heterostructurescan be made by the natural phase separation of the desired finalpolymers. In such heterostructures, the phases are not continuous fromone electrode to the other. Phase occlusions and polymer wetting layersinvariably occur during such phase separation and lead to trapping ofthe charge carriers, and hence less efficient light-emitting diodes andphotovoltaic diodes. Furthermore, the control of length scales of thephase separation within a bulk distributed heterostructure is limited(sub-micrometer length scales are difficult to obtain) and is dependenton the phase separation of the polymers.

These two approaches to generate micron-scale heterostructures inpolymer thin films do not provide enough control to form polymerheterostructures with the desired composition profile at will in boththe lateral and vertical directions. These approaches tend to lead tothe formation of occluded phases and wetting layers that are not alwaysdesirable.

A desirable morphology that cannot be readily formed, for example, is acolumnar nanostructure in which the desired charge-dissociationmaterials, and electron transporting and hole transporting materials arearranged appropriately in close proximity laterally, which is immenselybeneficial for OPVs. Similarly, a columnar nanostructure in which thedesired emissive material, and electron transporting and holetransporting materials are arranged appropriately in close proximitylaterally can be useful for OLEDs. Developing a method that can controlthe variation (or modulation) of the lateral composition profile of thematerials is hence desirable.

Another desired morphology is the graded composition profile in thevertical direction. A graded composition profile is one in which thecomposition of the materials varies systematically in the film thicknessdirection, from being rich in material A at the bottom face, to beingpoor in material A at the top face. The top face could further be boundby a film of pure material B. In other words, the heterostructureinterface becomes diffuse with respect to materials A and B. Thismorphology could be useful, for example, for OPVs in which the electrontransport material is graded into the hole transport material. Such avertical variation in composition may be considered as another form ofmodulated composition. Therefore, it is an objective of this inventionto provide heterostructures with such modulated composition profiles, amethod of making them, and devices incorporating them.

It is also an objective of this invention to create organic polymersemiconductor heterostructures on the sub-micron scale suitable for usein semiconductor devices, e.g. photovoltaic devices and light-emittingdiodes, which are at least as, and preferably more efficient than,existing polymer heterostructures and which preferably exhibit improvedperformance.

Further objectives of this invention include: (1) the creation oforganic solvent-processable heterostructures with one or more diffuse orabrupt interface; (2) the creation of abrupt nanostructures ofcontrolled vertical and horizontal dimensions; (3) the creation ofmulti-layer polymer heterostructures with high aspect ratios; (4) thecreation of tall polymer nanostructures that are self-organized withrespect to the underlying nanotemplates; and (5) new methods for makingthe structures.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided asupported polymer heterostructure, comprising a polymer immobilized on asubstrate and a second material deposited on said polymer.

The supported polymer heterostructure may provide a modulatedcomposition profile. For example, the modulated composition may be anin-plane columnar heterostructure, a vertically graded profile or anin-plane columnar heterostructure with a vertically graded profile.

The polymer heterostructure may be formed using one or more solvents.For example, the polymer heterostructure may be processable with anorganic solvent.

The polymer may be an organic polymer. The polymer may be immobilized onthe substrate by crosslinking.

The heterostructure, e.g. organic solvent-processable heterostructuremay contain one or more diffuse or abrupt interfaces, wherein theheterostructure may be nanostructured, and at least one of the materialsin the formation of the heterostructure can be crosslinked.

The heterostructure may be nanostructured, for example it may bein-plane nanostructured. The in-plane nanostructured heterostructure maycomprise a series of discrete regularly or randomly spaced elevations orregions of immobilised polymer. The elevations or regions may bereferred to as columns. The nanostructure may comprise a columnar orvertical lamellar or be of a mixed morphology.

The nanostructured elevations may have an average lateral distance space(i.e. the lateral length scale) from about 10 nm to about 1000 nm. Thenanostructured elevations may have a lateral length scale from about 10nm to about 300 nm, for example from about 10 nm to about 200 nm. Thenanostructured elevations may have a lateral length scale from about 20nm to about 1000 nm. The nanostructured elevations may have a laterallength scale from about 20 nm to about 200 nm.

The lateral length scale refers to the average distance from oneelevation to an adjacent elevation. This distance includes the totalwidth of an elevation and the void between said elevation and anadjacent elevation. The lateral length scale may be selected dependingon the nature of the device in which the heterostructure isincorporated.

For example, in OPVs, this length scale may be given approximately bytwice the exciton diffusion length, which is the distance over which theexciton has a half probability of decaying back to the ground state.This length scale may be in the range of from about 10 nm to about 50nm.

For OLEDs, this length scale may be given approximately by the Coulombcapture radius, which is the distance at which the electron and holeinteract with an electrostatic energy greater than the thermal energy.This length scale may be in the range of from about 30 nm to about 50nm.

The elevations may have a height from about 50 nm to about 500 nm. Theelevations may have a height from about 50 nm to about 200 nm.

The desired elevation height may be selected depending on the nature ofthe device in which the heterostructure is incorporated. For OPVs, thismay be given approximately by the thickness required to absorb theincoming light. For OLEDs, this may be given approximately by thethickness required for efficient recombination of the electrons andholes. For both OPV and OLED applications, the height is typically inthe range of from about 50 nm to about 200 nm.

The nanostructure may be formed by phase separation. The phaseseparation may be controlled by a phase control material. The phasecontrol material may be a low molecular weight polymer, oligomer, orpolyelectrolyte or ionomer.

In one embodiment, the heterostructure is nanostructured wherein thelateral length-scale is sub-0.1 μm with aspect ratio (height to widthratio) approaching or exceeding 1. The aspect ratio may be about orgreater than 1.

The nanostructure may be formed by phase separation of a two polymer-onesolvent system wherein the phase separation is dependent on the extentof miscibility between the two polymers, or the level of entropy ofmixing between the two polymers. A low entropy of mixing between thepolymers indicates that the polymers will phase separate. The polymerthat is used to direct the phase separation and which is notincorporated into the final heterostructure may be referred to as aphase control material.

The nanostructure may be formed by phase separation of a one polymer-twoliquid system, for example a one polymer-two solvent system wherein onesolvent has low entropy to the polymer i.e. is a poor solvent ornon-solvent which does not dissolve the polymer. The poor solvent ornon-solvent may be referred to as a phase control material and maycomprise any polar aprotic solvent or a high-boiling point alcohol whichdoes not dissolve the polymer.

The nanostructure may be formed by nano-imprinting of the first polymer.

One of the materials may be crosslinked by means of radiative or thermalcrosslinking methodologies.

The composition profile of the interface formed at the polymer andsecond material may be graded over a vertical distance of from about 5nm to about 500 nm. The grading may comprise 80-100% of the firstpolymer and 0-20% of the further material at the surface nearest thesubstrate to 80-100% of the further material and 0-20% of the firstpolymer at the opposite face of the heterostructure.

The heterostructure may further comprise one or more inorganiccomponents. The one or more inorganic components may be solventprocessable.

There is provided, in a second aspect a method of making a supportedheterostructure comprising an abrupt or diffuse interface, the methodcomprising or including the steps of:

-   -   (i) depositing on a substrate a first material together with a        phase control material to effect a phase separation between the        first material and the phase control material;    -   (ii) removal of the phase control material by a selective        solvent or by evaporation;    -   (iii) crosslinking the remaining deposited first material; and    -   (iv) deposition of a further material on at least the deposited        first material.

The first material may be a polymer. The phase control material may be apolymer. The phase control material may act as a sacrificial host. Thefirst material and the phase control material may be deposited in a twopolymer and one solvent system.

The phase control material may be a low molecular weight polymer,oligomer, or polyelectrolyte or ionomer.

The phase control material may be selectively removed using a selectiveor discriminating solvent which solubilizes the phase control materialbut not the first material.

The steps (i), (ii) and (iii) may be repeated at least once before step(iv). The method may further comprise use of a subsequent phaseseparation step employing the same two polymer and one solvent system tobuild up tall features with high aspect ratios.

The further material may be crosslinked.

The first material may be a polymer. The phase control material may be asolvent. The first material and the phase control material may bedeposited in a one polymer and two solvent system.

The phase control material may be a poor solvent or non-solvent, withlow entropy to the polymer. The solvent may comprise any polar aproticsolvent or a high-boiling point alcohol which does not dissolve thepolymer.

The method may further comprise use of a subsequent phase separationstep employing the same one polymer and two solvent system to build uptall features with high aspect ratios.

By depositing the further material over the crosslinked polymer in aback-filling process, a composite heterostructure that is bothvertically layered and also in-plane nanostructured may be obtained.

In one embodiment of the second aspect, the subsequent phase separationis self-aligned to the first determined pattern.

The further material may comprise a material with a lower molecularweight than the molecular weight of the polymer. Suitable materials mayinclude oligomers or small molecules.

According to a third aspect of the invention, there is provided a methodof making a supported heterostructure comprising a diffuse interface,the method comprising or including the steps of:

-   -   (i) depositing onto a substrate a first polymer;    -   (ii) crosslinking the polymer; and    -   (iii) depositing onto the first polymer a further material with        lower molecular weight than the molecular weight of the first        polymer, wherein the further material diffuses into the first        polymer to form the diffuse interface.

In (iii), the further material may be nanocrystals or nanowires. Themolecular weight of the nanocrystals or nanowires may be lower than orhigher than the molecular weight of the first polymer.

The composition profile of the interface may be graded over a verticaldistance of from about 5 nm to about 500 nm. The grading may comprise80-100% of the first polymer and 0-20% of the further material at thesurface nearest the substrate to 80-100% of the further material and0-20% of the first polymer at the opposite face of the hetero structure.

The heterostructure may comprise a vertical lamellar structure.

In one embodiment of the third aspect, the lower molecular weightmaterials are oligomers of organic semiconducting materials withappropriate band gap and/or charge-transport levels, such as thiophene,phenylene, fluorene, oxadiazole.

In another embodiment of the third aspect, the lower molecular weightmaterials are small molecules with the appropriate band gap and/orcharge-transport levels such as fullerene and its derivatives, andphthalocyanine.

In another embodiment of the third aspect, the lower molecular weightmaterials may be nanocrystals and nanowires of inorganic materials withthe appropriate band gap and/or charge-transport levels, such as metaloxides or Group II-VI compounds.

The band gap may be from about 1 eV to about 4 eV. The nanocrystals ornanowires may comprise a conduction band edge in the range of from about4.5 eV to about 2.5 eV and a valence band edge in the range of fromabout 5.5 eV to about 7.5 eV.

In another embodiment of the third aspect, the lower molecular weightmaterials are polymers with a degree of polymerisation between 2 to 10.

The substrate may be an electrode. Alternatively, an electrode may bedeposited on the substrate. The substrate or electrode may bepre-patterned prior to the first deposition. The pre-pattern may beapplied by surface-energy patterning or chemical patterning usingphotolithography, contact printing, or by in-plane phase separation of ablock polymer.

According to a fourth aspect of the invention, there is provided adevice made using the heterostructure according to the first aspect.

According to a fifth aspect of the invention, there are provided methodsof fabricating semiconductor devices, e.g. a semiconductor device, e.g.a photovoltaic device or a light emitting diode, comprising the methodsof the second and third aspects of the invention.

The device according to the fourth and fifth aspects may be aphotovoltaic device or a light-emitting diode.

For each of the various aspects of the invention, the polymers may beselected from classes of polymers such as poly(thiophenes), for examplepoly(alkylthiophenes) and copolymers thereof, poly(phenylene vinylenes)and copolymers thereof, poly(fluorenes), for examplepoly(octylfluorenes) and copolymers thereof, poly(alkylacrylates),polystyrene, poly(methyl methacrylate) and polyaniline.

For each of the various aspects of the invention the organic solventsmay be selected from classes of alcohols, esters, water, alkanes, andbenzenes.

For each of the various aspects of the invention, the substrate may beselected from any suitable planar substrate such as inorganic metal orsemiconductor wafers, glass and plastics.

In another embodiment of the second, third aspects, the deposition ofthe materials is by ink jet printing, nano-imprinting or spin-coating.

DETAILED DESCRIPTION OF THE INVENTION Substrates

Suitable substrates for use in the present invention, including themethods, include any planar substrates such as metal or semiconductor(e.g. inorganic) wafers, glass and plastics.

The substrate may be pre-patterned to further direct and enhance phaseseparation. One example of in-plane phase separation pre-patterning usesPMMA-co-PAAH di-block polymers. For example, a solution ofpoly(methylmethacrylate)-co-poly(acrylic acid) (PMMA-co-PAAH) isdeposited onto a substrate, and phase separates to give columns of PAAHapproximately 12 nm in diameter embedded in a PMMA matrix of width 20nm. Ion exchange of the PAAH with an alkali metal ion, for example, Na⁺to give a PAANa block which is insoluble in alcohols.

Irradiation at 254 nm followed by washing with isopropanol photolysesthe PMMA block and removes the degraded polymer, leaving thehydrophilic, ionic, PAANa on the surface. This allows the deposition ofthe first polymer. The first polymer may be cross-linked and renderedinsoluble before removal of the PAANa. A further material may thensubsequently be deposited.

By choosing the length scale and morphology of the underlyingpre-pattern, the in-plane nanostructure of the final heterostructure canbe further controlled.

Polymers

Classes of polymers which may be used as the first polymer in thepresent invention, including the methods according to the presentinvention, include those with the lowest unoccupied molecular orbital(LUMO) in the range of from about 1.9 eV to about 3.7 eV and the highestoccupied molecular orbital (HOMO) in the range of from about 4.8 eV toabout 6.2 eV. Classes of such polymers include poly(thiophenes) andcopolymers thereof, poly(phenylene vinylenes) and copolymers thereof,poly(fluorenes) and copolymers thereof, and polyaniline. These may beused as electron acceptors and hole acceptors. Examples of polymerssuitable for use in the present invention include but are not limited topoly(octylfluorenes) such aspoly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine),poly((9,9′-dioctylfluorene)-alt-(benzothiadiazole)) and[2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylenevinylene,poly((9.9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl)and poly(alkylthiophenes) such as poly(3-hexylthiophene).

Polymer Immobilization

The polymer may be immobilized by crosslinking. Crosslinking used inaccordance with the present invention, including the methods of thepresent invention, may be radiative or thermal. Where the polymer doesnot contain functional groups which allow crosslinking, the polymer maybe admixed with a suitable crosslinking agent.

The Further Material

If the immobilized polymer is selected to be the electron transportingcomponent of the device then the further material may comprise amaterial which may be used as a hole transporting material. Conversely,if the immobilized polymer is selected to be the hole transportingcomponent of the device then the further material may comprise amaterial which may be used as an electron transporting material.

The further material may comprise any polymer known to be effective as ahole transporting material or as an electron transporting material.

The further material may comprise a material with a lower molecularweight than the molecular weight of the polymer. Suitable materials mayinclude oligomers or small molecules.

Classes of oligomers which are suitable for use in the methods to creatediffuse interfaces include, but are not limited to, oligomers ofthiophene, phenylene, fluorene, oxadiazole.

Classes of small molecules used in the methods to create diffuseinterfaces include, for example, fullerene and its derivatives, andphthalocyanine. One example of a fullerene derivative is (6,6)-phenylC₆₁ butyric acid methyl ester.

Phase Control Material

Examples of materials suitable for use as phase control materialsinclude low molecular weight polymers, oligomers, polyelectrolytes orionomers, and also any solvent which does not dissolve the polymer whichis to form the nanostructure. The low molecular weight polymers may beselected from polymer families such as poly(alkylacrylates), polystyreneor poly(methyl methacrylate).

Preferred solvents for use as phase control materials include any polaraprotic solvent or a high-boiling point alcohol which does not dissolvethe polymer. One example of a polar aprotic solvent suitable for use inthe present invention is N,N-dimethylformamide. Suitable alcoholsinclude but are not limited to pentanol or ethylene glycol.

Methodologies

The method for forming a heterostructure, e.g. a columnar distributedheterostructure in accordance with the present invention may involve (i)use of a sacrificial “phase control” material which is removed after thephase-separation step to leave a pattern generated in one desiredmaterial, (ii) crosslinking of the material in the pattern generated,(iii) optionally, use of this generated pattern as the template for asubsequent phase separation step still employing the first material,which can be repeated, to build up tall high-aspect-ratio features, (iv)the deposition of a second material over the pattern, (v) optionally,crosslinking of the second material, and (vi) optionally, depositing athird material over the second, which can be repeated.

The nature of the second material may determine whether aheterostructure with an abrupt or diffuse interface is formed betweenthe two materials. The term diffuse interface is well known in the artand refers to an interface at which the components on opposite sides ofthe interface are compositionally graded on a length scale greater thanmolecular dimensions. The term abrupt interface is also well known inthe art and is one in which the components on opposite sides of theinterface form a molecularly distinct junction.

The present inventors have found approaches to create polymernanostructures with a range of phase separation length scales. Theseinclude phase separation in which the intended phase is phase separatedand connected directly to the intended electrode. This has a directimpact on the transport of charge carriers as all charges have a clearpath to or from its electrode.

In a two polymer one solvent system, a sacrificial host polymer (denotedpolymer B) acts as the phase control material and is phase separatedwith the first material (denoted polymer A). Polymer B is removed by adiscriminating solvent, leaving polymer A intact. A discriminatingsolvent is a solvent which selectively dissolves Polymer B while notdissolving polymer A.

Polymer A can be crosslinked using crosslinking methodologies to renderit insoluble for subsequent processing. Crosslinking of polymers mayoccur upon exposure to radiation, for example UV radiation, or uponthermal processing.

In the context of, for example, organic photovoltaic devices and organiclight-emitting diodes, a suitable polymer C can be depositedsubsequently to complete the nano-heterostructure devices.

The present inventors have also found a way to build polymernanostructures of high aspect ratios by a layer-by-layer approach. Thefirst phase separated nanostructure layer acts as a nanotemplate onwhich subsequent layers can self-organize, creating high aspect rationanostructures. A suitable polymer C can be deposited subsequently tocomplete a semiconductor device such as either an organic photovoltaicor organic light-emitting diode device.

If polymer A is selected to be the electron transporting component ofthe device then polymer C may comprise a polymer which may be used as ahole transporting material. Conversely, if polymer A is selected to bethe hole transporting component of the device then polymer C maycomprise a polymer which may be used as an electron transportingmaterial.

Besides utilizing the two polymer plus one solvent approach in thepreceding paragraph, the present inventors have also found that polymercolumnar distributed heterostructures can be created using a one polymerplus two solvent method.

In a one polymer plus one solvent system, the polymer will dissolve inthe solvent if the system has high enough entropy. Upon solutionprocessing of a one polymer dissolved in a one solvent system, thesolvent will evaporate and the polymer will be left behind.

A solvent with low entropy with the polymer (hence does not dissolve thepolymer) and higher vapor pressure than the solvent that is able todissolve the polymer can be mixed together in a one polymer plus twosolvent system assuming that the two solvents are miscible.

Upon solution processing, the solvent that dissolves the polymer willevaporate first, leaving behind the solvent with low entropy with thepolymer, hence the solvent will phase separate with the polymer. Whenthe second solvent dries, the polymer left will have nanostructures leftbehind by the solvent.

The method for forming a diffuse interface in a vertical heterostructuremay involve the following steps: (i) deposition of a first material;(ii) crosslinking of the first material; (iii) deposition of a secondmaterial which can infiltrate or diffuse into the first material.

Here we have found methods to diffuse or infiltrate material into thepolymer system with a range of materials. As a first step, this mayinclude depositing of a first material, for example, a polymer (A) ontothe intended electrode. This has a direct impact on the transport ofcharge carriers as all charges have a clear path to or from theelectrode. Polymer A can be crosslinked using crosslinking methodologiesto render it insoluble for subsequent processing. In the context oforganic photovoltaic and organic light-emitting diode devices, asuitable material can be subsequently deposited to complete thenano-heterostructure devices.

Deposition of materials may be achieved by contact printing,spin-coating, ink-jet printing or other solution methods known in theart.

Classes of solvents which are suitable for use in the methods includealcohols, esters, water, alkanes, and benzenes.

Nanostructures

In the present invention, the morphology of the nanostructuredheterostructure may be controlled by use of phase control materials asoutlined previously. Deposition of a polymer and a phase controlmaterial, followed by phase separation of the polymer and the phasecontrol material and subsequent removal of the phase control materialmay result in a series of discrete elevations, or columns, of polymer ona first surface of the substrate and separated by voids left by thephase control material.

Upon deposition of a further material, the voids may be backfilled bythe further material, resulting in a supported heterostructurecomprising alternate regions of the polymer and the further material.For the heterostructure to be suitable for use in a semi-conductingdevice, the deposition or backfilling may be such that the discreteelevations of the polymer on the substrate are substantially orcompletely encapsulated by the further material.

The lateral direction referred to herein runs parallel to orsubstantially parallel to the first surface of the substrate. The heightor thickness of the elevations refers to the dimension which isperpendicular to or substantially perpendicular to the plane of thesubstrate.

Uses of the Organic Polymer Nanoscale Heterostructures

The methodologies described herein outline the formation of organicpolymer nanoscale heterostructures. Such heterostructures are suitablefor use in the fabrication of polymer organic semiconductor devices suchas organic light emitting diode devices or organic photovoltaic devices.

In the construction of such devices, the polymer in question (Polymer A)may be deposited onto a substrate or an electrode. The electrode mayalso be pre-patterned by methods known in the art, for example,surface-energy patterning or chemical patterning using photolithography,contact printing or in-plane phase separation of a block polymer.Following formation of the heterostructure, a second electrode may bedeposited onto the opposite surface of the heterostructure to the firstelectrode to complete fabrication of the device.

The anode may be formed from a transparent layer of Indium Tin Oxidecoated with a layer of conducting poly(3,4-ethylenedioxythiophene) dopedwith poly(styrenesulfonate). The cathode, usually a metal with a lowwork function such as calcium, aluminium or magnesium, may be depositedby thermal or electron beam evaporation of the metal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly and without limitation, with reference to the accompanying drawingsand the following Examples, in which:

FIGS. 1a and 1b describe general schemes for the formation of nanoscaleheterostructures in accordance with the present invention;

FIG. 2 shows the photovoltaic performance of devices according to thepresent invention compared to known blended composites; and

FIG. 3 shows the dielectric function of a composition formed inaccordance with the fourth aspect of the present invention.

With reference to FIG. 1, a solution (1) comprising a first polymer anda phase control material is deposited onto a substrate (2). Phaseseparation into distinct phases of polymer (3) and phase controlmaterial (4) then occurs. In a third step, the phase control material(4) can be selectively removed through use of a solvent which dissolvesthe phase control material (4) but does not dissolve the first polymer(3) or in the case of the phase control material being a solvent, byevaporation, leaving behind a series of voids and nanostructuredelevations of polymer (3), which may then be immobilized bycrosslinking. As a fourth step, deposition of a second material (5) ontothe first polymer (3) completes fabrication of the heterostructure.

In order to increase the height of the nanostructure, crosslinkedpolymer (3), immobilized on substrate (2), may undergo a subsequentdeposition of solution (1) using the first layer as a template, as shownin FIG. 1b . This intermediate structure would then be processed asbefore: selective removal of phase control material (4) followed bycrosslinking, and deposition of a second material (5) to back-fill intofirst polymer (3).

As shown in FIG. 1, the deposition of the second material (5) may besuch that it completely or substantially encapsulates the first polymer(3) upon the substrate (2). The steps of (i) deposition of the firstpolymer (3) and phase control material (4) and (ii) removal of the phasecontrol material (4) and crosslinking of the first polymer (3) may berepeated as many times as is necessary, depending on the requiredmorphology of the heterostructure.

Dimension (a) in FIG. 1 refers to the lateral length scale of theelevations of deposited polymer. For convenience, this is indicated asbeing the sum of the width of one elevation and the width of theneighbouring void. Dimension (b) refers to the height or thickness ofthe elevations. Dimension (c) refers to the total height or thickness ofthe supported heterostructure.

As can be seen in FIG. 2, in comparison with the bulk distributedheterostructure devices given by the bi-blend of PFB and F8BT (maximumphoton to electron conversion efficiency of 1.5% at 475 nm), it is clearthat the columnar heterostructures of the present invention give muchbetter performance (up to 7% at the same wavelength) by a factor ofgreater than 4.5.

In summary, a natural phase separation between polymers or solvent andpolymer may be achieved according to the present invention. Tall polymernanostructures can be built using the layer by layer upon polymernanotemplate approach. Also a diffuse interface may be achievedaccording to the present invention. The methods according to the presentinvention are more easily processable than processes that requirelithography. The methods according to the present invention are alsosolution processable which enables large area production by usingtechniques such as ink jet printing.

EXAMPLES Materials

All chemicals and reagents were obtained from Sigma-Aldrich unlessotherwise stated. (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) wasobtained from Nano-C.

PFB and F8BT may be obtained, for example, from American Dye Sources.

Test Methods Quantum Efficiency

External quantum efficiencies were measured in an inert atmosphere or ina vacuum (10⁻⁶ Torr) using a calibrated system comprising a solarsimulator (Oriel), monochromator (Cornerstone) and sourcemeter (Keithley2400). Short circuit current and open circuit voltage were measuredusing a semiconductor parametric analyzer (Keithley 4200). Incidentlight power was measured with a calibrated silicon diode (OPT301M).Quantum efficiencies were obtained as a function of wavelength bycomparing the photocurrent output of the device under test with that ofa calibrated photodiode. Power conversion efficiencies (PCE) werecalculated according to Equation 1:

$\begin{matrix}{{P\; C\; E} = \frac{{Fill}\mspace{14mu} {Factor} \times {Voltage}_{({{open}\mspace{11mu} {circuit}})} \times {Current}_{({{short}\mspace{11mu} {circuit}})} \times 100}{{Incident}\mspace{14mu} {Power}}} & (1)\end{matrix}$

wherein the fill factor is the ratio of the actual maximum obtainablepower to the theoretical power, given as a percentage.

Measurement of Nanostructures

The morphology of the nanostructures were measured by atomic forcemicroscopy (Digital Instruments Dimension 3000 atomic force microscopein tapping mode).

All film thicknesses are measured on a Tencor P2 profilometer.

Ellipsometry

Ellipsometry measurements were carried out on a JA Woollam M2000Vellipsometer. Variable angle spectroscopic ellipsometry was measured atboth the top and bottom interfaces, wherein the top interface refers tothe air-polymer interface and the bottom interface refers to thepolymer-glass interface. Del and Psi ellipsometric values were collectedas a function of wavelength for angles of 50, 55 and 65 degrees.

Optical modelling was carried out using WVASE32 modelling software.Optical constants n and k were fitted by using the measured thickness.

Example 1a

The polymerpoly(9,9′-dioctylfluorene-co-bis-N,N′(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine)PFB with molecular weight ˜100,000 is dissolved in toluene at aconcentration of 9 mg/ml. Polystyrene (PS) with molecular weight ˜10,000is dissolved in toluene at a concentration of 9 mg/ml. The two solutionsare added in equal amounts such that the weight ratio of the twopolymers are equal. A photo-crosslinker is added at 6 wt % 12 mm by 12mm silicon oxide wafers are pre-cleaned with RCA standard clean 1solution for 15 minutes. The solution is then spin coated onto the waferat 3000 revolutions per minute (rpm) for 40 seconds. The thickness ofthe resulting film is ˜40 nm. Methyl ethyl ketone (MEK) is dropped ontothe film for 10 seconds to selectively remove the polystyrene. The waferis then spun dry of methyl ethyl ketone with dissolved polystyrene at3000 rpm for 40 seconds. The remaining polymer PFB reveals ananostructure with feature sizes in the range of 100 nm to 500 nm, morespecifically 300 nm. The PFB layer is then cross-linked by exposing thesample to ultra-violet light (254 nm) for 2 mins under nitrogen atambient temperature (10 ppm O₂/H₂O).

Example 1b

Poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine)PFB with molecular weight ˜300,000 gmol⁻¹ is dissolved in toluene at aconcentration of 9 mg/mL. Polystyrene (PS) with a molecular weight˜10,000 gmol⁻¹ is dissolved in toluene at a concentration of 9 mg/mL.PFB is the desired first polymer to be nanostructured and is also thehole acceptor, e.g. for use in a photovoltaic cell. PS is the phasecontrol material. The two solutions are mixed in equal amounts such thatthe weight ratio of the two polymers is 1:1. Bisazide photo-crosslinkerethylene bis(4-azido-2,3,5-trifluoro-6-isopropylbenzoate) is dissolvedin toluene at a concentration of 9 mg/mL and added to the polymersolution mixture at 1 wt % with respect to the total polymer weight.

The solution is spin-coated onto clean ITO-glass substrates pre-coatedwith a 50 nm thick poly(3,4-ethylenedioxythiophene):poly(styrenesulfonicacid) (PEDT:PSSH) at 3000 revolutions per minute (rpm) for 40 seconds.The thickness of the resultant film is 40 nm. Methyl ethyl ketone (MEK)is dropped onto the film for 10 seconds and spun-off to selectivelyremove the polystyrene.

The film remaining on the substrate is PFB (with dispersed crosslinker)in-plane nanostructured at a lateral length scale of ˜100 nm andthickness of 20 nm.

The nanostructured PFB layer is then cross-linked by exposing the sampleto UV light (254 nm) for 2 minutes under a nitrogen atmosphere (<1 ppmO₂/H₂O).

Poly((9,9′-dioctylfluorene)-alt-(benzothiadiazole)) (F8BT), the electronaccepting material for the photovoltaic cell, is then spun-cast from a15 mg/mL solution in toluene to give a film thickness of about 50 nm.

After the crosslinking step in the two polymers plus one solventapproach, the same solution of (PFB/PS) is spin coated onto the polymernanotemplate. The PFB will align to the PFB nanotemplate below. Thepolystyrene is then selectively removed using MEK. The resultingself-organized polymer nanostructure is rendered insoluble by exposureof The PFB layer can be cross-linked by exposing the sample toultra-violet light (254 nm) for 2 mins under nitrogen ambient (10 ppmO₂/H₂O). Third and subsequent layers can be added by repeating thesteps.

Example 1c

As described in the preceding paragraph, in a variation of the method ofExample 1b, the steps of deposition of the PFB/PS containing solution,washing with MEK and crosslinking were carried out a second time priorto deposition of the F8BT solution, resulting in a nanostructured PFBlayer with a thickness of 35 nm.

Example 1d

In a variation of the method of Example 1b, the steps of deposition ofthe PFB containing solution, washing with MEK and crosslinking wererepeated twice prior to deposition of the F8BT solution, resulting in ananostructured PFB layer with a thickness of 50 nm.

The external quantum efficiencies of Examples 1b, 1c and 1d areillustrated in FIG. 2.

Example 1e

The polymerpoly(9,9′-dioctylfluoreneco-bis-N,N′(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine)PFB with molecular weight ˜100,000 is dissolved in toluene at aconcentration of 9 mg/ml. Polystyrene (PS) with molecular weight ˜10,000is dissolved in toluene at a concentration of 9 mg/ml. The two solutionsare added in equal amounts such that the weight ratio of the twopolymers are equal. A photo-crosslinker is added at 6 wt %. 12 mm by 12mm ITO substrate after 10 min oxygen plasma was span with PEDT:PSSH at5000 revolutions per minute (rpm) for 40 seconds to give 40 nm thickPEDT:PSSH. The solution is then spin coated onto this PEDT:PSSH film at3000 rpm for 40 seconds. The thickness of the resulting film is ˜40 nm.Methyl ethyl ketone (MEK) is dropped onto the film for 10 seconds toselectively remove the polystyrene. The wafer is then spun dry of methylethyl ketone with dissolved polystyrene at 3000 rpm for 40 seconds. Theremaining polymer PFB reveals a nanostructure with feature sizes ˜100nm. The PFB layer is then cross-linked by exposing the sample toultra-violet light (254 nm) for 2 mins under nitrogen ambient (10 ppmO₂/H₂O).

Example 1f

The polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene(OC1C10-PPV) with molecular weight ˜400,000 is dissolved in toluene atconcentration of 9 mg/ml. Polystyrene (PS) with molecular weight ˜40,000is dissolved in toluene at a concentration of 9 mg/ml. The two solutionsare added in equal amounts such that the weight ratio of the twopolymers are equal. A photo-crosslinker is added at 6 wt %. 12 mm by 12mm silicon oxide wafers are pre-cleaned with RCA standard clean 1solution for 15 minutes. The solution is then spin coated onto the waferat 3000 revolutions per minute (rpm) for 40 seconds. The thickness ofthe resulting film is ˜40 nm. Methyl ethyl ketone (MEK) is dropped ontothe film for 10 seconds to selectively remove the polystyrene. The waferis then spun dry of methyl ethyl ketone with dissolved polystyrene at3000 rpm for 40 seconds. The remaining polymer PFB reveals ananostructure with feature sizes ˜100 nm. The PFB layer is thencross-linked by exposing the sample to ultra-violet light (254 nm) for 2mins under nitrogen ambient (10 ppm O₂/H₂O).

Example 1g

Polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene(OC1C10-PPV) with molecular weight ˜1,000,000 gmol⁻¹ is dissolved intoluene at a concentration of 5 mg/mL. Polystyrene (PS) with molecularweight ˜40,000 is dissolved in toluene at a concentration of 9 mg/mL.The two solutions are added in equal amounts such that the weight ratioof the two polymers are equal. Bisazide photo-crosslinker ethylenebis(4-azido-2,3,5-trifluoro-6-isopropylbenzoate) is dissolved inchlorobenzene at a concentration of 9 mg/mL and added to the polymersolution mixture to give 1 wt % with respect to the total polymerweight. The remaining steps are identical to those in Example 1b.

Example 1h

Polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene(OC1C10-PPV) with molecular weight ˜100,000 is dissolved in toluene atconcentration of 9 mg/ml. Polystyrene (PS) with molecular weight ˜10,000is dissolved in toluene at concentration of 9 mg/ml. The two solutionsare added in equal amounts such that the weight ratio of the twopolymers are equal. A photo-crosslinker is added at 6 wt %. 12 mm by 12mm ITO substrate after 10 min oxygen plasma was spun with PEDT:PSSH at5000 revolutions per minute (rpm) for 40 seconds to give 40 nm thickPEDT:PSSH. The solution is then spin coated onto this PEDT:PSSH film at3000 rpm for 40 seconds. The thickness of the resulting film is ˜40 nm.Methyl ethyl ketone (MEK) is dropped onto the film for 10 seconds toselectively remove the polystyrene. The wafer is then spun dry of methylethyl ketone with dissolved polystyrene at 3000 rpm for 40 seconds. Theremaining polymer PFB reveals a nanostructure with feature sizes ˜100nm-500 nm. The PFB layer is then cross-linked by exposing the sample toultra-violet light (254 nm) for 2 mins under nitrogen ambient (10 ppmO₂/H₂O).

Example 2a

The polymerpoly(9,9′-dioctylfluoreneco-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine)PFB with molecular weight ˜100,000 is dissolved in toluene at aconcentration of 9 mg/ml. 6 wt % of a photo-crosslinker is added next. Adissimilar solvent (dimethylformamide) which has low entropy to PFB isadded at 5 vol %. The solution is spin coated onto pre-cleaned siliconoxide wafers at 3000 rpm at 40 seconds. PFB is naturally phase separatedwith length scales in the order of a few micrometers. The PFB layer canbe cross-linked by exposing the sample to ultra-violet light (254 nm)for 2 mins under nitrogen ambient (10 ppm O₂/H₂O).

Tall polymer nanostructures can also be built using the one polymer andtwo solvent approach as the nanotemplate layer and subsequent layers canbe built using either the two polymer one solvent or one polymer twosolvent approach.

Example 3a

Polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene(OC1C10-PPV) with molecular weight 100,000 is dissolved in toluene at aconcentration of 9 mg/ml. 6 wt % of a photo-crosslinker is added next.The solution is spin coated onto pre-cleaned silicon oxide wafers at3000 rpm at 40 seconds to give a 55 nm thick film. The OC1C10-PPV layercan be cross-linked by exposing the sample to ultra-violet light (254nm) for 2 mins under nitrogen ambient (10 ppm O₂/H₂O). (6,6)-phenylC₆₁-butyric acid methyl ester (PCBM) with molecular weight ˜910.9 isdissolved in chlorobenzene. PCBM is spin-coated twice onto OC1C10-PPVlayer at 3000 rpm at 40 seconds to give a 35 nm thick OC1C10-PPV/14 nmPCBM.

Example 3b

Polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene(OC1C10-PPV) with molecular weight 100,000 is dissolved in toluene at aconcentration of 9 mg/ml. 6 wt % of a photo-crosslinker is added next.The solution is spin coated onto 40 nm thick PEDT:PSSH at 3000 rpm at 40seconds to give a 55 nm thick film. The OC1C10-PPV layer can becross-linked by exposing the sample to ultra-violet light (254 nm) for 2mins under nitrogen ambient (10 ppm O₂/H₂O). (6,6)-phenyl C₆₁-butyricacid methyl ester (PCBM) with molecular weight ˜910.9 is dissolved inchlorobenzene. PCBM is spin-coated twice onto OC1C10-PPV layer at 3000rpm at 40 seconds to give a 35 nm thick OC1C10-PPV/14 nm PCBM.

Example 3c

Polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene(OC1C10-PPV) with molecular weight 100,000 is dissolved in toluene at aconcentration of 9 mg/ml. 6 wt % of a photo-crosslinker is added next.The solution is spin coated onto 40 nm thick PEDT:PSSH at 3000 rpm at 40seconds to give a 55 nm thick film. The OC1C10-PPV layer can becross-linked by exposing the sample to ultra-violet light (254 nm) for 2mins under nitrogen ambient (10 ppm O₂/H₂O). (6,6)-phenyl C₆₁-butyricacid methyl ester (PCBM) with molecular weight ˜910.9 is dissolved inchlorobenzene to give final concentrations between 0.1 mg/mL and 10mg/mL. The PCBM solution is inkjet-printed over the crosslinked(OC1C10-PPV) over multiple passes to give a layer of (OC1C10-PPV) ofapproximately 35 nm in thickness on top of which is deposited a layer ofPCBM of approximately 30 nm thickness.

Example 3d

Poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene (OC1C10-PPV)with molecular weight 1,000,000 gmol⁻¹ was dissolved in toluene at aconcentration of 3 mg/mL. Bisazide photo-crosslinker ethylenebis(4-azido-2,3,5-trifluoro-6-isopropylbenzoate) was added to themixture to give a final crosslinker concentration of 0.5 w/w % withrespect to the OC₁C₁₀-PPV. This solution was then spin coated onto cleanITO-glass substrate pre-coated with a 50 nm thick PEDT:PSSH film at 3000rpm for 40 seconds to give a 55 nm thick film.

The OC₁C₁₀-PPV layer was then crosslinked by exposing to UV light (254nm) for 2 minutes under a nitrogen atmosphere (<1 ppm O₂/H₂O) and rinsedwith toluene on the spinner. (6,6)-phenyl C₆₁ butyric acid methyl ester(PCBM) (molecular weight 910.9 g mol⁻¹) was dissolved in chlorobenzeneto a concentration of 9 mg/mL. PCBM was then infiltrated into thepolymer layer by spinning at 3000 rpm for 40 seconds.

In this example, the PCBM is the electron acceptor and electrontransporter, while the OC₁C₁₀-PPV is the hole acceptor and holetransporter of the photovoltaic cell. The primary light absorber isOC₁C₁₀-PPV, which has an absorption coefficient five times greater thanthat of PCBM at wavelengths between 450 and 550 nm.

Infiltration of PCBM into the OC₁C₁₀-PPV film results in a diffuseinterface and a graded composition, as confirmed by top-side andbottom-side spectroscopic ellipsometry, the results of which are shownin FIG. 3. The dielectric function determined from the top-side(air-polymer) shows a higher PCBM content compared to the bottom-side(polymer-substrate). This can be seen from the relatively higher kvalues at wavelengths below 450 nm, where the OC₁C₁₀-PPV absorptiontails off and the strong absorption band of PCBM begins. Theellipsometry results also suggest the possible presence of a thin layerof PCBM (about 3 nm) on the surface of the graded composite film.

Optical modeling suggests that the average PCBM:OC₁C₁₀-PPV volume ratioin the film is between 0.5:1 to 1:1 with slightly higher PCBM content atthe top (air-polymer) interface than at the bottom.

Example 3e

Following infiltration of PCBM in Example 3c, an aluminium metal cathodewas deposited onto the heterostructure to give a photodiode pixel.

Semiconductor devices made from such heterostructures exhibit higherefficiencies than conventional blend devices.

1. A method of making a supported heterostructure comprising a diffuseinterface, the method comprising or including the steps of: i)depositing on a substrate a first polymer; ii) crosslinking the polymer;and iii) depositing on the first polymer a further material with lowermolecular weight than the molecular weight of the first polymer, whereinthe further material diffuses into the first polymer.
 2. The method ofclaim 1, wherein the lower molecular weight material comprises anoligomer.
 3. The method of claim 2, wherein the oligomer comprises anoligomer of thiophene, phenylene, fluorene or oxadiazole.
 4. The methodof claim 1, wherein the lower molecular weight material comprises asmall molecule.
 5. The method of claim 4, wherein the small moleculecomprises fullerene or a derivative thereof.
 6. The method of claim 1,wherein the lower molecular weight material comprises one or morenanocrystals or nanowires.
 7. The method of claim 1, wherein thesubstrate is pre-patterned.
 8. The method of claim 1, comprising thefurther steps of incorporating the supported heterostructure in asemiconductor device.
 9. The method according to claim 1, wherein thedevice is a photovoltaic device.
 10. The method according to claim 1,wherein the device is a light-emitting diode.